February 2007 Air Repair OBDII Review 19
How Important is Wiring
From the July 2005 issue of Air Repair.
By Pat Weber, Owner, Weber Automotive, Glenview, IL
A frustrated customer came into my shop with an OBDII
fail because of no communication. Another shop had been
unable to get it to communicate. Because I had little repair
history on this vehicle, a 1996 Jeep, I asked the customer
for a history. He did inform me that the vehicle had been
in an accident. He had very little problems with the vehicle
and it was well-maintained. I’m glad I found out the
vehicle had been in an accident as I wouldn’t have even
suspected it. The body job was one of the best I have ever
seen, and I probably wouldn’t have thought to look for oth-er
damage that could have an effect on an emissions test.
Upon further investigation, I found that the body shop
missed a few important items such as missing ground
connections and wiring harness damage. So I spent a few
hours fixing damaged and ungrounded wiring.
The result was we got the vehicle to pass the test after
another repair facility was unable to find the problems.
The lesson learned from this experience is that spending
time with the customer to get a history of the vehicle can
give clues as to how to go about approaching the problem.
Knowing that the vehicle had been in an accident, we
checked for ground and power, which are needed to es-tablish
communication. As OBDII vehicles age, accidents
and a vehicle’s wiring will be areas that will need to be
Damaged wiring affecting 1996 Jeep OBDII test. checked more thoroughly.
Pin #5 is the sensor signal ground and may not be used on
all vehicles. Check your wiring diagram for confirmation.
The voltage reading you get (while cranking) on Pin #5 will
depend on the amount of resistance across the computer and
should be close to what you see on Pin #4. Pin #16 should
have system (battery) voltage with the engine running.
The next step is to verify the data signal with a DSO.
(The data transmission rate occurs so quickly that a mul-timeter
will not see the signal. Even if it does recognize it
in “peak detect” mode, the value is averaged). Connect the
DSO between signal and ground or Bus+ and Bus-. The
signal will resemble a square waveform pattern (Figure 2)
and should be 5 to 7 volts high, and may require a time base
less than 100 microseconds per division to recognize the
signal as being present. This is done with the engine run-ning.
The pins that are used for this will vary depending on
the manufacturer and the protocol that is used. The protocol
that is used can usually be determined by what pins are pres-ent
in the OBD II connector. There are four protocols we are
concerned with presently. The four protocols are:
J1850 VPW (variable pulse width) (10.4kbps)
The VPW system is used on most GM vehicles and
sends data out on Pin #2 (SAE J1850 line (Bus+)) only.
This system is also used on some Fords and Chryslers
and may use Pin #10.
J1850 PWM (pulse width modulation) (41.6kbps)
The PWM system is used on most Ford vehicles and
sends data out on Pin #2 (SAE J1850 line (Bus+))
and Pin #10 (SAE J1850 line (Bus-)) This signal will
be inverted on Pin #10 and is not used on all PWM
systems.
ISO 9141-2 (International Standards Organization)
(10.4kbps)
KWP2000 (Key Word Protocol) (Newer Version of ISO
9141)
The ISO and KWP systems are similar. The ISO system
is used on most Chryslers and Imports. The KWP
system is used on some Imports. These systems send
data out on Pin #7 (K line of ISO 9141-2 & KWP 2000).
Pin #15 may also be a part of this system (L line of ISO
9141-2 & KWP 2000). Pin #15 is not always used.
The following web sites may be helpful in making a deter-mination
of the type of protocol that is being used on the
vehicle you are working with:
http://www.scantool.net/scanned_list.htm
http://www.obd-2.com/dlc.html this site also has
OBDII connector locations.
http://www.obd-2.de/dl/probs.txt
http://www.etools.org/files/public/generic-protocols-
04-1-04.htm
Thanks to George Generke, Al Santini and John Thorton for the help
in verifying the information in this article.
How To Check A Vehicle That Won’t Communicate
Continued from page 18.
